Methods for converting data which comprise the outlines of image parts ("objects") to raster data are known. The raster is built up from a number of scan lines which can be divided into parts and stored in an apparatus make up of a central unit with at least two parallel processors, each provided with a local bitmap memory subdivided for raster memory lines in which the corresponding scan lines can be stored. An example of a multiprocessor system is disclosed in French Patent Application No. 2,561,009. In another system, a communication channel interconnects a central unit and the processors.
An apparatus of this latter type forms part of a raster image processor as used in printers and visual display units to which data are fed from a processing station concerning the characters or figures (objects) to be printed, in the form of outline descriptions, such as vector descriptions. The printer (e.g., a laser printer) is adapted to deflect a modulated beam of light line by line over a light-sensitive surface. By advancing the light-sensitive surface perpendicularly to the direction of deflection it is possible to expose a complete page imagewise and print it on a receiving sheet by a known method. The outline descriptions must therefore first be converted to raster data.
The raster data are stored in a page-size bitmap memory. The position of each image point on the light-sensitive surface corresponds to the position of a memory element in the memory where the information of that image point is stored. To be able to embody a high-speed printer it is necessary to use a high-speed raster image processor. One example of such a raster image processor is described in WO 87/02159.
This raster image processor is based on a parallel architecture. Each image processor processes a permanently allocated part (local memory) of this bitmap memory. The conversion of an object is effected on the basis of this local memory permanently allocated to each subprocessor. In such a processor, each subprocessor receives only that part of the object which is associated with the local bitmap memory belonging to that subprocessor.
The computing task required to fill that local bitmap memory is effected by the subprocessor physically connected to that local bitmap memory.
In the conversion of an object in a multiprocessor system, it is possible to distinguish three activities:
(a) transmission of information about the object to the processors, PA1 (b) calculation of the raster memory lines to be filled in the local memories, and PA1 (c) filling of the raster memory lines with a pattern of bits corresponding to that part of the object which was intended for that part of the bitmap memory.
Steps (b) and (c) are performed by the same subprocessor only if the filling of the bitmap memory must be effected via that processor. Accordingly, irrespective of how the raster memory lines of the bitmap memory are distributed over the available subprocessors, there are objects which have a shape such that not every subprocessor is equally involved in the raster conversion of that object. This results in inefficient processor use because there are, on average, a number of processors that are motive inactive.
The object of the invention is to achieve more efficient processor use.